Plotting spectrum of OFDM

I am using following code for plotting spectrum of OFDM.

Hs=spectrum.mtm(30); 
Pxx=psd(Hs,IFFT_output(:),'Fs',nFFTSize);
plot(Pxx.frequencies+1,10*log10((Pxx.data)),'b');

size of IFFT_output is 128x100 and size of nFFTSize is 128. It works fine. But when I add Cyclic prefix of 32 samples, the size of IFFT_output becomes 160x100. To plot the spectrum of this new variable, should I change nFFTSize.

Answer 1

[Albatross]

To plot the spectrum of the (OFDM modulated time domain signal + cyclic prefix) you need to change nFFTsize to 160. You can zero pad to 256 if necessary to speed up the fft and then plot the spectrum. To demodulate the received signal correctly, you need to strip out the cyclic prefix first before the fft.

Answer 2

[Amit Kansal]

In this line of code

Pxx=psd(Hs,IFFT_output(:),'Fs',nFFTSize);

nFFTSize represents the sampling frequency (FS) and so, yes, its value should be changed when adding the cyclic prefix to teh OFM modulated signal (128 to 160).